Noise evidence where the only plant is a heat pump
An air source heat pump is assessed by a fixed desk calculation under permitted development and against measured background noise when permission is needed. Where the two tests diverge, and the siting decisions that matter most.
Heat pump noise evidence planning work is unusual because the test changes completely depending on whether you need permission. Install an air source heat pump under Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 and the noise test is a fixed limit calculated by a prescribed method. Need planning permission instead, and the assessment moves to BS 4142, which compares the pump against the measured background sound level and is a harder test to pass in a quiet location.
Knowing which regime you are in before specifying the unit saves a great deal of trouble, because a pump that comfortably satisfies one can fail the other on the same garden wall.
When heat pump noise becomes a planning matter
Under permitted development, noise is not assessed by anybody at the council. It is a condition of the right. The installation has to meet the planning standard for domestic air source heat pumps, and if it does not, the installation is not permitted development and needs an application. Nobody checks at the time, which is why the evidence only surfaces when a neighbour complains two winters later.
Where permission is required the position reverses. The case officer consults environmental health, environmental health asks for a noise assessment, and the assessment is against the background sound level rather than against a fixed figure.
Permission is required in several common situations: the property is a flat rather than a house, the building is listed or within the curtilage of a listed building, the site is a scheduled monument, the installation falls outside the Part 14 conditions, or the pump serves a commercial building. Designated land carries its own restrictions on where the unit may be placed.
One caution on the Order itself. The conditions in Part 14 for air source heat pumps have been amended more than once, including changes in England that relaxed the restrictions on unit size, number and proximity to a boundary. Check the current wording on the legislation site rather than relying on a figure quoted in an older article or installer leaflet.
The calculation route used for domestic units
The planning standard for permitted development installations uses a desk calculation rather than a measurement, which is the only practical approach for an installation that does not yet exist.
The threshold is a sound pressure level of 42 dB(A) at the assessment position, which is a point one metre from the centre of the nearest window or door of a habitable room in a neighbouring property. The calculation starts from the manufacturer's declared sound power level for the specific model, then applies:
- Distance attenuation from the unit to the assessment position.
- A correction for reflective surfaces adjacent to the unit, because a pump in a corner with two hard surfaces beside it is several decibels louder at the receptor than the same pump in open ground.
- A barrier correction where a solid obstruction interrupts the line of sight.
The output is a single figure that either sits at or below 42 dB(A) or does not. There is no judgement in it, which makes it fast and also makes it blunt.
What installers get wrong is the input. The declared sound power level must be the one from the standardised test for that exact model and, where the manufacturer publishes separate figures, the one for the relevant operating mode. A figure for a different capacity in the same product range is not a substitute, and quiet mode figures should not be used unless quiet mode will actually run continuously.
Boundary position and the assessment location
The assessment position is a neighbour's habitable room window or door, not the boundary line. That distinction decides most marginal cases.
A unit placed two metres from a boundary fence with the nearest neighbouring window 12 metres beyond it is in a much better position than one placed six metres from a boundary with a bedroom window three metres on the other side. Measure to the window.
Four siting decisions that change the result more than the choice of unit:
- Avoid internal corners. Each additional hard reflective surface next to the unit adds to the radiated level, and a pump tucked into the angle between a rear wall and a return wall is the worst available position.
- Keep the discharge direction away from neighbouring windows, since directivity matters even where the calculation treats the source as omnidirectional.
- Avoid mounting on a lightweight wall backing onto a bedroom, because structure-borne transmission is not captured by the calculation at all and is the single most common cause of complaints within the same dwelling.
- Put distance between the unit and the fence rather than relying on the fence as a barrier, since a barrier only helps where it breaks the line of sight and has enough mass.
Cumulative effect of several units
The calculation method deals with one installation at a time. It has nothing to say about a terrace where six households install pumps over three years, which is exactly the situation now arriving in older housing stock.
Cumulative assessment needs the BS 4142 route, comparing the combined rating level of all the units against the measured background. Several pumps add logarithmically, so four identical units produce roughly 6 dB more than one, and the character correction for a tonal fan is applied to the combination.
For a scheme of new dwellings with a pump each, do this properly at application stage. A Noise Impact Assessment modelling the full set at the nearest existing receptors costs far less than retrofitting enclosures after occupation, and it is the question an environmental health officer will ask about a terrace of air source heat pumps on a tight plot.
The gap between the two tests
This is the honest problem with the current arrangement and it is worth stating plainly.
| Permitted development route | Planning application route | |
|---|---|---|
| Test | Fixed limit of 42 dB(A) | Rating level compared with background |
| Location | Neighbour's habitable room window or door | Nearest noise-sensitive receptor |
| Background noise | Not considered | Central to the assessment |
| Tonal character | Not corrected for | Correction applied |
| Who checks | Nobody, unless complained about | Environmental health officer |
In a suburban garden with a night background around 40 dB, a pump calculated at 40 dB(A) is unremarkable. In a rural garden with a night background of 25 dB, the same pump would be well above background under BS 4142 and would attract an adverse rating, yet it satisfies the permitted development condition and no application is needed.
That gap is lawful and it is also where complaints come from. A pump that meets the permitted development standard can still be a statutory nuisance, and the environmental health route under the Environmental Protection Act 1990 remains open to a neighbour regardless of planning status. Specifying for the quietest realistic condition rather than for the threshold is the sensible response.
Mitigation, enclosures and siting
In descending order of effectiveness per pound spent: move the unit, increase the distance to the neighbour's window, remove a reflective surface beside it, fit an acoustic barrier close to the unit that breaks the line of sight and has reasonable mass with no gaps at the base, mount on anti-vibration feet onto a concrete pad rather than a wall bracket, and oversize the unit slightly so that it runs at lower fan speed for more of the year.
Proprietary acoustic enclosures work but they restrict airflow, and restricting airflow on a heat pump reduces efficiency and can cause the unit to work harder, which is self-defeating. Any enclosure has to come from the manufacturer or be approved by them.
Remember the defrost cycle. A pump that is unobtrusive at steady state can be noticeably louder when it reverses to defrost in cold damp weather, which is precisely when windows are shut and nobody notices, or at six in the morning when they do.
What to submit with an application
For a permitted development installation, keep the file even though nothing is submitted: the manufacturer's sound power data sheet for the exact model, the completed calculation showing the distance, the reflective surface correction and the result, and a dimensioned site plan showing the unit and the assessment position. If a complaint arrives, that file is the whole defence.
For a planning application, the pack is: a site plan and elevations showing the unit, a BS 4142 assessment with a background measurement at the correct period, the manufacturer's octave band data, the mitigation specified, and a short Planning Statement dealing with the design and the low carbon benefit. Where the pump is part of a wider scheme the heating strategy belongs in the Energy Statement rather than being repeated in two documents with different figures.
Browse all 66 report types on PlanScope and start a draft for a heating or plant installation.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect noise assessments to be prepared by a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What noise limit applies to a domestic air source heat pump?
Under permitted development the planning standard sets a sound pressure level of 42 dB(A) at a point one metre from the centre of the nearest window or door of a habitable room in a neighbouring property. The figure is calculated from the manufacturer's declared sound power level with corrections for distance, reflective surfaces and barriers.
Is the noise limit measured at the boundary?
No, at the neighbour's habitable room window or door. A unit two metres from a fence with the nearest window twelve metres beyond it is in a far better position than one six metres from a boundary with a bedroom window three metres on the other side. Always measure to the window.
Why does a compliant heat pump still generate complaints?
Because the permitted development test is a fixed limit that ignores background noise and tonal character. In a rural garden with a night background of 25 dB, a pump calculated at 40 dB satisfies the condition but would rate badly under BS 4142. A statutory nuisance route remains open to neighbours regardless of planning status.
How are several heat pumps in a terrace assessed?
The permitted development calculation handles one installation at a time and says nothing about cumulative effect. A combined assessment uses BS 4142, adding the units logarithmically so four identical pumps produce roughly 6 dB more than one, with the character correction applied to the combination.
Do acoustic enclosures work on heat pumps?
They can, but they restrict airflow, which reduces efficiency and can make the unit work harder. Any enclosure should come from the manufacturer or be approved by them. Moving the unit, increasing distance to the neighbour's window and removing a reflective surface beside it are usually more effective per pound spent.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.